Highly Bright and Photostable Li(Gd,Y)F₄:Yb,Er/LiGdF₄ Core/Shell Upconversion Nanophosphors for Bioimaging Applications

Intense green-emitting Li(Gd,Y)F₄:Yb,Er/LiGdF₄ core/shell (C/S) upconversion nanophosphors (UCNPs) with a tetragonal bipyramidal morphology are synthesized. The morphology and UC luminescence of the Li(Gd,Y)F₄:Yb,Er UCNPs are significantly affected by the Li precursors, and bright UC green-emitting Li(Gd,Y)F₄:Yb,Er UCNPs with a tetragonal bipyramidal shape, i.e., UC tetragonal bipyramids (UCTBs), are synthesized using LiOH·H₂O as a Li precursor. A LiGdF₄ shell is grown on the Li(Gd,Y)F₄:Yb,Er UCTBs, and the C/S UCNPs exhibit 4.7 times higher luminescence intensity than core UCTBs. The C/S UCNPs show a high absolute UC quantum yield of 4.6% under excitation with 980 nm near infrared (NIR) light, and the UC luminescence from the C/S UCNPs is stable under continuous irradiation with the 980 nm NIR laser for 1 h. The hydrophobic surfaces of the as-synthesized C/S UCNPs are modified to hydrophilic surfaces by using poly(acrylic acid) (PAA) for bioimaging applications. They are applied to human cervical adenocarcinoma (HeLa) cell imaging and SK-MEL-2 melanoma cell imaging and in vivo imaging, including subcutaneous and intramuscular imaging, and UC luminescence images with high signal-to-noise ratio are obtained. Furthermore, sentinel-lymph-node imaging is successfully conducted with the PAA-capped Li(Gd,Y)F₄:Yb,Er/LiGdF₄ C/S UCNPs under illumination with NIR light.